This invention relates in general to sewing machines and in particular to a new and useful sewing machine counterbalanced needle bar drive.
In conventional sewing machines having their needle bar drive designed as a crank drive, a counterbalance weight firmly secured to the armshaft of the machine is provided rotating in opposition to the crankpin (U.S. Pat. No. 2,128,120). The counterweight is selected to equilibrate the rotating masses, namely the crank arm with the crank pin and a portion of the needle bar link, and partly the vertical first order inertia forces which are caused by the needle bar and a portion of the needle bar link. This partial compensation, however, is a compromise justifiable only for low-speed to medium speed sewing machines, since along with a partial compensation for the vertical inertia forces, a horizontal component develops, so that a rotary inertia force vector is produced varying in magnitude. In high speed sewing machines, such non-compensated inertia forces cause strong vibrations of the machine housing adversely affecting the operation.
U.S. Pat. No. 3,318,274 discloses a sewing machine having features which include a counterbalance drive in which the crank pin connected through a link to a needle bar forms a part of a gimbal mounting. As is well known, a gimbal suspension produces a purely sinusoidal motion of the needle bar. In addition to the usual counterbalance weight connected directly to the armshaft, a second counterbalance weight is provided which is connected to the driver of the gimbal mount and rotates in the opposite direction. Due to the two counterbalance weights the inertia forces which are produced by the sinusoidal drive of the needle bar and are of first order only, become completely balanced. Since the two counterweights rotate in opposite directions, their horizontal force components are always mutually opposed and compensate each other.
In contradistinction to sinusoidally moved needle bars, needle bars driven through a crank have a longer period of dwell at the upper point of reversal, so that a longer interval is available for the feed of the work. Along with lower costs of crank drives, this is a reason for designing needle bar drives mostly as crank drives, not as gimbal ring drives.
It is known that in a needle bar drive designed as a crank drive, also second order inertia forces are produced, in addition to the first order ones. The magnitude of these second order inertia forces is proportional, among others, to the square of the angular velocity and to the ratio of crank radius-to-length of the needle bar link. Solutions to the equilibration of inertia forces of first and second order in crank drives are known in the field of internal combustion engines, for example from the book "Die Dynamik der Verbrennungskraftmaschine (Dynamics of Internal Combustion Engines) by H. Schron, Springer Publication, Vienna 1942, page 43. The solution shown there in FIG. 16 however, is very expensive and requires much space and is therefore unsuitable for sewing machines.